There is a pressing need in industry for a monitoring system that is capable of identifying and quantifying trace quantities of hazardous gasses escaping into the air. The monitors, located in the vicinity of processing plants and storage facilities, provide early warning of impending danger to plant personnel and the public and enable corrective action to be taken in time to avoid disaster. An ideal monitoring system is capable of operating unattended continuously for extended periods of time without the need for frequent maintenance or calibration. Monitoring systems in present use rely on an electrochemical cells as the sensing element. These systems fall short of the ideal in that they often lack adequate specificity, require frequent maintenance for calibration and replenishment of electrolyte, and are limited to operation at ambient temperatures above 0.degree. C., because of freezing of the electrolyte.
The Ion Mobility Spectrometer (IMS) is an accepted analytical tool capable of identifying and quantifying trace amounts of a substance in a sample. Basically, an IMS comprises an analyzer cell, means for ionizing samples of an analyte admitted to the cell and means for determining the times required for the ions of the various substances present in the cell to traverse a specific length of the cell under the influence of an accelerating electric field and against the force of a stream of drift gas flowing through the cell in a direction opposite to that of the electric field. A stream of purified air may be used as a carrier gas to introduce the analyte sample into the cell and a stream of purified air may also be used as the drift gas. Both the carrier gas and the drift gas are therefore readily available at an installation site in unlimited quantities and no maintenance is required of the sensor other than the occasional replacement of filters for purifying the carrier and drift gasses. An IMS therefore appears to be the ideal sensor for use in a monitoring system.
However, it has been found that an IMS operated in a conventional manner, using air as the carrier and drift gasses, lacks the specificity necessary to detect many of the acid gasses of interest, such as hydrogen fluoride, hydrogen chloride, nitrogen dioxide, and others. The reason for such lack of specificity is that the ion peak characteristic of pure air alone and the ion peak characteristic of the analyte gas in air both arrive at the ion detector of the IMS at virtually the same times. Since the existence of an alarm condition is determined by the amplitude of the ion current detected at a specific arrival time, the pure air ion peak cannot be distinguished from the analyte and air ion peak, when the analyte is an acid gas.
It is an object of the invention to provide a system for monitoring the atmosphere to determine the presence of hazardous gasses therein.
It is another object of the invention to provide such an atmospheric monitoring system which is capable of operating unattended for extended periods of time, without the need for frequent maintenance or calibration, and which will automatically generate an alarm upon the detection of a potentially dangerous condition.
It is a further object of the invention to provide a monitoring system having the above-mentioned attributes and which is specifically responsive to acid gasses as analytes.
It is still another object of the invention to provide a method for operating an Ion Mobility Spectrometer (IMS) which improves the specificity of the IMS for the detection of acid gasses and therefore enables the use of an IMS as a sensor in a system for monitoring the atmosphere for the presence hazardous acid gasses.
It is a specific object of the invention to provide a method for operating an IMS which improves the specificity of the IMS for the detection of hydrogen fluoride.
Other objects and advantages of the invention will become evident as a complete understanding thereof is gained from the detailed description of the invention to follow, and the accompanying drawings.